For the need of high-precision spectral radiance measurement, a holographic plane grating double monochromator with the double decked Ebert-Fastie structure is designed. This system consists of two spherical collimating mirrors, a plane mirror, a roof mirror, two holographic plane gratings, entrance slit, exit slit and mid slit. Wavelength region from 160nm to 400nm can be scanned. This double decked design has a compact structure, high spectral resolution and low stray level. Its spectral resolution is less than 0.15nm, and stray level is 10^-6, which satisfies the need of high-precision spectral radiance measurement in ultraviolet wavelength region in space.